In the search for high sensitivity and direct atmospheric sampling of trace species, techniques have been developed such as atmospheric sampling, glow-discharge ionization (ASGDI), atmospheric pressure ionization (API), electron capture detection (ECD) and negative-ion chemical ionization (NICI) that are capable of detecting parts-per-billion to parts-per-trillion concentrations of trace species, including explosive, in ambient air. These techniques are based on positive or negative ion formation via charge transfer to the target, or electron capture under multi-collision conditions in a Maxwellian distribution of electron energies [with a peak at about 40 millielectron volts (meV)] at the source temperature (300 K). Subsequent detection of the ion-molecule reaction products or the electron-attachment products is carried out by using time-of-flight, quadrupole, magnetic-sector, ion-trap or analog-current measurement methods.
One drawback of the high-pressure, corona- or glow-discharge devices is that they are susceptible to interferences either through indistinguishable product masses, or through undesired ion-molecule reactions. The ASGDI technique is relatively immune from such interferences, since at target concentrations of &lt;1 ppm the majority of negative ions arises via electron capture rather than through ion-molecule chemistry. A drawback of the conventional ECD, and possibly of the ASGDI, is that they exhibit vanishingly small densities of electrons with energies in the range 0-10 meV, as can be seen from a typical Maxwellian electron energy distribution function at T=300 K. Higher electron currents are needed at low (&lt;10 meV) energies.
Slowing the electrons to these subthermal (&lt;10 meV) energies is crucial because the cross section for attachment of several large classes of molecules (including the explosives, chlorohalocarbon compounds and perfluorinated carbon compounds) is known to increase to values larger than 10.sup.-12 cm.sup.2 at near-zero electron energies. In fact, in the limit of zero energy, these cross sections are predicted to diverge as .epsilon..sup.-1/2, where .epsilon. is the electron energy. This is a direct consequence of the Wigner threshold law for electron attachment.
To provide a better "match" between the electron energy distribution function and the attachment cross section, a new concept of attachment in an electrostatic mirror was developed referred to hereinafter as the electron reversal technique. In that technique, electrons were brought to a momentary halt by reversing their direction with electrostatic fields. At a reversal region R, the electrons have zero or near-zero energy. A beam of target molecules is introduced, and the zero or near-zero energy electrons are attached to the molecules of the beam. The resultant negative ions may then be easily extracted. This basic electron reversal technique has been improved by Mark T. Bernius and Ara Chutjian as described in U.S. Pat. No. 4,933,551 to allow for better reversal geometry, higher electron currents, lower backgrounds and increased negative-ion extraction efficiency.
In the application of the electron reversal technique to detection of the molecules of explosives RDX, PETN, and TNT by negative-ion formation under single-collision conditions, the fact that these molecules are known to attach thermal-energy electrons is exploited for detection of trace species, but for zero-energy electrons higher electron current is needed for the electron reversal technique. Improvements in this regard by the present invention permits a factor of about 25 increase in detection sensitivity for these classes of zero electron-energy attaching molecules, including the explosives.
The electron reversal technique is a new analytical tool which differs in several significant ways from other methods. Because this technique builds up electron density in the energy region of maximum attachment cross section, attachment (ionization) efficiencies are expected to be high. Indeed, the sensitivity of this technique to the detection of Cl.sup.- ions from CCl.sub.4 has been measured to be 10 pptr with a counting rate of 900 Hz. Neither attachment cross sections nor rate constants for the explosives are available. Assuming values comparable to CCl.sub.4, this would give a sensitivity of the electron reversal technique in the design of the present invention of pptr (90 Hz) to explosives.
Unlike the ASGDI, API, ECD or NICI techniques, negative-ion generation by electron reversal is also able to access resonance at .epsilon.&gt;0, beyond the range of thermalized energies. This is accomplished by shifting the location of the electron turning point with respect to the target beam. Furthermore, because measurements are carried out under single-collision conditions, there is no secondary ion-molecule chemistry. Finally, by detecting product masses, this electron reversal technique is capable of identifying one or more "signature" ions in the attachment process. In applications where time is not critical, the use of several mass detectors to detect products concurrently would be feasible. This could mitigate strongly against interferences, reduce false alarms, and could even identify directly which type(s) of explosives are being detected.
Notwithstanding developments in other prior art techniques, there exists a need in trace-species analysis detectors that are sensitive, specific, and resistant to interferences from nontargeted chemical components. The reversal electron technique utilizes the fact that there are many classes of molecules which have negative-ion resonances at low energies (below about 5 eV) and hence can form one or more negative ions upon electron attachment. However, the sensitivity of such a device will depend upon, among other factors, the space-charge limited electron current at the resonance energy that can be delivered to the reversal region. Consequently, improvement in the electron current source is important. This is especially critical for explosives detection where the explosives vapor pressures are extremely low (10.sup.-9 torr for RDX, for instance).